Carbonaceous substrates, and more specifically carbon-carbon composites are widely used in the aerospace and aviation industries due to the physical and mechanical properties of such materials. The high strength-to-weight ratio of carbon-carbon composites provides weight savings to aircraft components. This characteristic, coupled with the mechanical strength, toughness and dimensional stability of carbon-carbon composites to temperatures in excess of 4000.degree. F. makes these materials ideal for aerospace applications.
A potentially limiting characteristic of carbon-carbon composites is that such materials tend to rapidly oxidize at temperatures above 800.degree. F. in oxygen-containing environments. The degradation of the material due to oxidation causes loss of mechanical and physical properties. This problem must be prevented in order for these materials to be effectively used for aerospace applications, such as turbine engine exhaust nozzles and the space shuttle leading edge, where temperatures often exceed 2000.degree. F. for extended periods of time.
One solution to the problem of carbonaceous substrate degradation at elevated temperatures is to coat the substrate surface with SiC. A silicon carbide coating effectively protects the carbonaceous substrate from degradation by providing a barrier to oxygen diffusion. Coating the material with silicon carbide may be accomplished by pack cementation, chemical vapor deposition or other viable processing techniques.
U.S. Pat. Nos. 4,544,412 and 4,476,164 teach a powdered pack mixture and a pack cementation process used to produce SiC conversion coatings on carbon-carbon materials. The pack mixture defined consists of roughly 60% SiC powder, 30% Si powder, 1% B powder and 9% Al.sub.2 O.sub.3 powder.
U.S. Pat. No. 4,465,777 also discloses a boron-containing pack mixture composition for use in the protection of carbon-based materials. The pack contains from 25% to 40% Si, from 50% to 70% SiC, and from 1% to 15% B, with a minor amount of MgO, usually as an impurity in the boron. This reference further discloses the use of filter paper and filter paper pulp release agents. Impregnation of the substrate with tetraethylorthosilicate and/or heat treatment prior to the cementation process in an attempt to increase coating pick-up is also disclosed.
U.S. Pat. No. 4,585,675 relates to pack cementation formulation of silicon carbide protective coatings for carboncarbon substrates. This patent teaches a powdered pack mixture composition of 40%-50% Si, 30%-50% SiC and 20%-30% Al.sub.2 O.sub.3 that is reacted with the surface of a carbonaceous substrate to provide a primary SiC coating for protection against the degradation of carbon-carbon substrates at high temperatures. An enhancement coating, used to seal the primary coating, is also disclosed. This patent further discusses a hand tamped packing method and an alternative time saving process in which a slurry form of the reactive pack mixture is poured around the part in a graphite retort.
Relatively recent technology improvements in the formulation and densification of carbon-carbon substrates have made it increasingly difficult to form a silicon carbide coating thereon with established pack cementation techniques. To enable coating of some of the newer carbon-carbon substrates, more reactivity is required of the pack mixture.
Further, difficulty is typically encountered in removing the pack mixture from the coated part surface after cementation particularly when boron is a pack mixture ingredient. Attempts have been made by others to circumvent the problem by keeping the boron content of the pack mixture extremely low, i.e. less than about 1%, or by using carbonaceous substances such as filter paper or filter paper pulp as a pack release agent. The filter paper is deficient in that it shrinks and cracks away from the part surface, directly exposing it to the reactive pack ingredients and resulting in pack sticking. Also, parts with complex configurations are difficult to wrap, making filter paper a poor alternative for use on such articles. Because of its fibrous nature, filter paper pulp tends to clog the spray devices used to apply it to the part surface. A less expensive, more readily sprayable alternative material for a release agent is needed to address these problems.
Given the foregoing drawbacks, what is needed is a pack mixture composition that increases the degree of silicon carbide coating formation. Further, a means of ensuring that such compositions will easily release from the substrate after pack cementation is also needed.
It is, therefore, one object of the present invention to provide a pack mixture composition for coating carbonaceous substrates which will impede the degradation of the substrate by oxidation at high temperatures.
It is another object of the present invention to provide a means for the clean release of expended pack mixture from the substrate.
These and other objects of the present invention will become apparent to those skilled in the art in the description of the invention that follows, and in the examples and claims appended hereto.